In order to fabricate photovoltaic cells on a larger scale, and in particular for solar panels, one solution consists in using a composite ribbon made up of a carbon ribbon covered on both faces in respective layers of polycrystalline silicon. The carbon ribbon passes continuously through a bath of molten silicon, preferably vertically. On leaving the bath, both faces of the ribbon are covered in a relatively thin layer of silicon. This produces a composite silicon-carbon-silicon ribbon. This method is known as RST (Ribbon on Sacrificial Template) and is described in various patents, e.g. FR 2 386 359, FR 2 550 965, and FR 2 568 490. It can be used to obtain layers of silicon having thickness that is as small as 50 micrometers (μm). Nevertheless, such thin layers are fragile and therefore difficult to handle. That is why the layers as fabricated in this way generally have thicknesses that are greater than 150 μm.
The composite ribbons are cut into composite plates of small size (e.g. 12.5 centimeters (cm) by 12.5 cm). These plates are then heated in a gas containing oxygen to a temperature close to 1000° C. in order to burn off the carbon ribbon. This operation, referred to as “burning off” is described for example in patent FR 2 529 189. Starting from each composite plate, this produces two thin plates of silicon having the same dimensions as the composite plates, i.e. small dimensions. Thereafter, the silicon plates are subjected to various treatments leading to the implementation of photovoltaic cells, these treatments differing depending on the type of cell that is to be fabricated. In general, after burn-off, the front and rear faces are deoxidized, junctions are formed by diffusing a precursor over at least one of the faces, an ant reflection layer is deposited on the front face, and electric contacts are deposited.
That plate fabrication method is a discontinuous method, well suited to fabricating plates of small dimensions that are relatively thick (thicknesses greater than 150 μm), and it can be incorporated well in present technology for fabricating photovoltaic cells made from plates of crystalline silicon.
Nevertheless, in order to make it economically attractive to obtain photovoltaic electricity by using the crystalline silicon technique, it is desirable to provide photovoltaic cells that are very thin (thicknesses lying in the range 30 μm to 100 μm) and that present high photovoltaic conversion efficiency.
In this context, the above method is confronted with the critical problem of manipulating thin plates having thickness of less than 150 μm. The multiple manipulation operations that are performed on such plates of large dimensions that are fragile because they are thin and that present high levels of residual stress, and that are performed at high rates of throughput (more than 1000 units per hour), lead to greatly reduced fabrication yield.